1. Field of the Invention
The present invention relates to an ultrasonic blood flow imaging apparatus which utilizes the ultrasonic Doppler effect to acquire blood flow information of the inside of a body to be examined, or a target body, and displays this information as a two-dimensional image.
2. Description of the Related Art
An ultrasonic blood flow imaging apparatus, used together with the ultrasonic Doppler method and pulse echo method, generates blood flow data and tomographic image (B mode) data by means of a single ultrasonic probe and supplies both data, superimposed one on another, to a monitor to provide color display of a superimposed image of the blood flow profile and tomographic image on the monitor.
This ultrasonic blood flow imaging is based on the following principle.
When an ultrasonic beam is irradiated onto a living body in which blood is flowing, the intermediate frequency fc of this beam is disturbed by flowing blood cells, and the intermediate frequency is shifted by frequency fd due to the resultant Doppler effect. The frequency f of the ultrasonic echo having received the Doppler effect becomes f=fc+fd. The frequencies fc and fd are expressed by the following equation: EQU fd=2v.multidot.cos.theta./c.multidot.fc (1)
where v is the blood velocity, .theta. is an angle defined by the ultrasonic beam and a blood vessel, and c is the speed of sound.
Accordingly, the blood velocity v can be attained by detecting the Doppler shift frequency fd.
In measuring the blood velocity using the Doppler effect as described above, radiation of an ultrasonic pulse in a given direction on a living body from an ultrasonic transducer is repeated several times. Ultrasonic echoes from the living body that have undergone the Doppler effect are received by the ultrasonic transducer and are sequentially converted into echo signals. The echo signals are input to a phase detector to detect a Doppler shift signal. In this case, Doppler shift signals are detected for, for example, 256 sampling points in the raster direction (depth direction of a target body). The Doppler shift signal detected for each sampling point is subjected to frequency analysis in a frequency analyzer, and is converted into a scan signal by a DSC (Digital Scan Converter) to be two-dimensionally displayed as a color flow mapping (CFM) image on a monitor.
In displaying the Doppler shift signal as a CFM image on the monitor, the mean blood velocity is expressed in terms of an angle (+.pi. to -.pi.) or a frequency (+fr/2 to -fr/2). The range of these angular and frequency expressions corresponds to a color range, namely, blue to red with black in between. The term fr is the rate frequency of an ultrasonic pulse.
According to conventional apparatuses, smoothing of a CFM image on the time axis (persistence), or time smoothing of the CFM image, is executed, as shown in FIG. 10. In FIG. 10, X(s) indicates an input signal, Y(s) an output signal, ##EQU1## is a transfer function of a prior system for time-smoothing the image, CFM is a differential operator, a is a coefficient, TOF is a one-frame period of the CFM image, f.sub.OF is a frequency of one frame of the CFM image, and f is a frequency according to the variation of the graduation of the input signal X(s). The time smoothing is a method to correlate data at the same coordinates (x, y) in a case where a plurality of sequential CFM images are formed for one frame period (T.sub.OF). A circuit for executing this processing is, theoretically a cyclic digital filter, and is, physically a low-pass filter along the time axis. The effect of using this filter is illustrated in FIG. 11. With f.sub.OF being the frame frequency of a CFM image and f being a change in data between adjoining frames, when f/f.sub.OF &gt;1/2 is satisfied, gradation change or an aliasing phenomenon occurs. Unless the frame frequency f.sub.OF is sufficiently greater than the change f in image data, not only is there no smoothing effect, but also the accurate image data sharply changes, thus providing an improper or undesired image.